Knee
The knee joint joins the thigh with the leg and consists of two articulations: one between the femur and tibia, and one between the femur and patella. It is the largest and most complicated joint in the human body.Kulowski (1932), p 618 The knee is a mobile trocho-ginglymus (i.e. a pivotal hinge joint),See trochoid and ginglymus. which permits flexion and extension as well as a slight medial and lateral rotation. Since in humans the knee supports nearly the entire weight of the body, it is the joint most vulnerable both to acute injury and the development of osteoarthritis. Human anatomy The knee is a complex, compound, condyloid variety of a synovial joint. It actually comprises three functional compartments: the femoropatellar articulation consists of the patella, or "kneecap", and the patellar groove on the front of the femur through which it slides; and the medial and lateral femorotibial articulations linking the femur, or thigh bone, with the tibia, the main bone of the lower leg.Burgener (2002), p 390 The joint is bathed in synovial fluid which is contained inside the synovial membrane called the joint capsule. Upon birth, a baby will not have a conventional knee cap, but a growth formed of cartilage. In human females this turns to a normal bone knee cap by the age of 3, in males the age of 5. Articular bodies The articular bodies of the femur are its lateral and medial condyles. These diverge slightly distally and posteriorly, with the lateral condyle being wider in front than at the back while the medial condyle is of more constant width.Platzer (2004), p 210 The radius of the condyles' curvature in the sagittal plane becomes smaller toward the back. This diminishing radius produces a series of involute midpoints (i.e. located on a spiral). The resulting series of transverse axes permit the sliding and rolling motion in the flexing knee while ensuring the collateral ligaments are sufficiently lax to permit the rotation associated with the curvature of the medial condyle about a vertical axis.Platzer (2004), pp 194-195 The pair of tibial condyles are separated by the intercondylar eminence composed of a lateral and a medial tuberclePlatzer (2004), p 202. The patella is inserted into the thin anterior wall of the joint capsule. On its posterior surface is a lateral and a medial articular surface, both of which communicate with the patellar surface which unites the two femoral condyles on the anterior side of the bone's distal end.Platzer (2004), p 192 Articular capsule The articular capsule has a synovial and a fibrous membrane separated by fatty deposits. Anteriorly, the synovial membrane is attached on the margin of the cartilage both on the femur and the tibia, but on the femur, the suprapatellar bursa or recess extends the joint space proximally. Behind, the synovial membrane is attached to the margins of the two femoral condyles which produces two extensions similar to the anterior recess. Between these two extensions, the synovial membrane passes in front of the two cruciate ligaments at the center of the joint, thus forming a pocket direct inward. Platzer (2004), p 210 Bursae Numerous bursae surround the knee joint. The largest communicative bursa is the suprapatellar bursa described above. Four considerably smaller bursae are located on the back of the knee. Two non-communicative bursae are located in front of the patella and below the patellar tendon, and others are sometimes present. Menisci The articular disks of the knee-joint are called menisci because they only partly divide the joint space.Platzer (2004), p 26 These two disks, the medial meniscus and the lateral meniscus, consist of connective tissue with extensive collagen fibers containing cartilage-like cells. Strong fibers run along the menisci from one attachment to the other, while weaker radial fibers are interlaced with the former. The menisci are flattened at the center of the knee joint, fused with the synovial membrane laterally, and can move over the tibial surface. Platzer (2004), p 208 The menisci serve to protect the ends of the bones from rubbing on each other and to effectively deepen the tibial sockets into which the femur attaches. They also play a role in shock absorption, and may be cracked, or torn, when the knee is forcefully rotated and/or bent. Ligaments The ligaments surrounding the knee joint offer stability by limiting movements and, together with several menisci and bursae, protects the articular capsule. Intracapsular The anterior cruciate ligament (ACL) stretches from the lateral condyle of femur to the anterior intercondylar area The ACL is critically important because it prevents the tibia from being pushed too far anterior relative to the femur. It is often torn during twisting or bending of the knee. The posterior cruciate ligament (PCL) stretches from medial condyle of femur to the posterior intercondylar area. Injury to this ligament is uncommon but can occur as a direct result of forced trauma to the ligament. This ligament prevents posterior displacement of the tibia relative to the femur. The transverse ligament stretches from the lateral meniscus to the medial meniscus. It passes in front of the menisci. Is divided into several strips in 10% of cases. The two menisci are attached to each others anteriorly by the ligament.Diab (1999), p 200 The posterior and anterior meniscofemoral ligaments stretch from posterior horn of lateral meniscus to the medial femoral condyle. They pass posteriorly behind the posterior cruciate ligament. The posterior meniscofemoral ligament is more commonly present (30%); both ligaments are present less often. The meniscotibial ligaments (or "coronary") stretches from inferior edges of the mensici to the periphery of the tibial plateaus. Extracapsular The patellar ligament connects the patella to the tuberosity of the tibia. It is also occasionally called the patellar tendon because there is no definite separation between the quadriceps tendon (which surrounds the patella) and the area connecting the patella to the tibia.MedicineNet.com, Definition of Patellar tendon This very strong ligament helps give the patella its mechanical leverageMoore (2006), p 194 and also functions as a cap for the condyles of the femur. Laterally and medially to the patellar ligament the lateral and medial patellar retinacula connect fibers from the vasti lateralis and medialis muscles to the tibia. Some fibers from the iliotibial tract radiates into the lateral retinaculum and the medial retinaculum receives some transverse fibers arising on the medial femoral epicondyle. The medial collateral ligament (MCL a.k.a. "tibial") stretches from the medial epicondyle of the femur to the medial tibial condyle. It is composed of three groups of fibers, one stretching between the two bones, and two fused with the medial meniscus. The MCL is partly covered by the pes anserinus and the tendon of the semimembranosus passes under it. It protects the medial side of the knee from being bent open by a stress applied to the lateral side of the knee (a valgus force). The lateral collateral ligament (LCL a.k.a. "fibular") stretches from the lateral epicondyle of the femur to the head of fibula. It is separated from both the joint capsule or the lateral meniscus.. It protects the lateral side from an inside bending force (a varus force). Lastly, there are two ligaments on the dorsal side of the knee. The oblique popliteal ligament is a radiation of the tendon of the semimembranosus on the medial side, from where it is direct laterally and proximally. The arcuate popliteal ligament originates on the apex of the head of the fibula to stretch proximally, crosses the tendon of the popliteus muscle, and passes into the capsule. Movements The knee permits flexion and extension about a virtually transversal axis, as well as a slight medial and lateral rotation about the axis of the lower leg in the flexed position. The knee joint is called "mobile" because the femur and menisci move over the tibia during rotation, while the femur rolls and glides over the menisci during extension-flexion.Platzer (2004), pp 212-213 The center of the transverse axis of the extension/flexion movements is located where both collateral ligaments and both cruciate ligaments intersect. This center moves upward and backward during flexion, why the distance between the center and the articular surfaces of the femur changes dynamically with the decreasing curvature of the femoral condyles. The total range of motion is dependent of several parameters such as soft-tissue restraints, active insufficiency, and hamstring tightness. Extended position With the knee extended both the lateral and medial collateral ligaments, as well as the anterior part of the anterior cruciate ligament, are taut. During extension, the femoral condyles glide into a position which causes the complete unfolding of the tibial collateral ligament. During the last 10° of extension, an obligatory terminal rotation is triggered in which the knee is rotated medially 5°. The final rotation is produced by a lateral rotation of the tibia in the non-weight-bearing leg, and by a medial rotation of the femur in the weight-bearing leg. This terminal rotation is made possible by the shape of the medial femoral condyle, assisted by the iliotibial tract and is caused by the stretching of the anterior cruciate ligament. Both cruciate ligaments are slightly unwinded and both lateral ligaments become taut. Flexed position In the flexed position, the collateral ligaments are relaxed while the cruciate ligaments are taut. Rotation is controlled by the twisted cruciate ligaments; the two ligaments get twisted around each other during medial rotation of the tibia — which reduces the amount of rotation possible — while they become unwounded during lateral rotation of the tibia. Because of the oblique position of the cruciate ligaments at least a part of one of them is always tense and these ligaments control the joint as the collateral ligaments are relaxed. Furthermore, the dorsal fibers of the tibial collateral ligament become tensed during extreme medial rotation and the ligament also reduces the lateral rotation to 45-60°. Blood supply The femoral artery and the popliteal artery help form the arterial network surrounding the knee joint (articular rete). There are 6 main branches: * 1. Superior medial genicular artery * 2. Superior lateral genicular artery * 3. Inferior medial genicular artery * 4. Inferior lateral genicular artery * 5. Descending genicular artery * 6. Recurrent branch of anterior tibial artery The medial genicular arteries penetrate the knee joint. Disorders and injury In sports that place great pressure on the knees, especially with twisting forces, it is common to tear one or more ligaments or cartilages. An increasingly common victim to injury is the anterior cruciate ligament(ACL), often torn as a result of a rapid direction change while running or some other, violent twisting motion. It can also be torn by extending the knee forcefully beyond its normal range. In some such cases, other structures incur damage as well. Especially debilitating is the unfortunately common "unhappy triad" of torn medial collateral and anterior cruciate ligaments and a torn medial meniscus. This typically arises from a combination of inwards forcing and twisting. Before the advent of arthroscopy and arthroscopic surgery, patients having surgery for a torn ACL required at least nine months of rehabilitation. With current techniques, such patients may be walking without crutches in two weeks, and playing some sports in but a few months. In Australian rules football, knee injuries are among the most common, especially in ruck contests, involving the crashing of two knees during the leap. These injuries forced new rule changes for the 2005 season. In addition to developing new surgical procedures, ongoing research is looking into underlying problems which may increase the likelihood of an athlete suffering a severe knee injury. These findings may lead to effective preventive measures, especially in female athletes, who have been shown to be especially vulnerable to ACL tears from relatively minor trauma. There are disorders of the knee which are not necessarily the result of injury, for example patellofemoral syndrome and arthritis. Diagnosis Several diagnostic maneuvers help clinicians diagnose an injured ACL. In the anterior drawer test, the examiner applies an anterior force on the proximal tibia with the knee in 90 degrees of flexion. The Lachman test is similar, but performed with the knee in only about twenty degrees of flexion, while the pivot-shift test adds a valgus (outside-in) force to the knee while it is moved from flexion to extension. Any abnormal motion in these maneuvers suggests a tear. The diagnosis is usually confirmed by MRI, the availability of which has greatly lessened the number of purely diagnostic arthroscopies performed. Animal anatomy In humans the knee refers to the joints between the femur, tibia and patella. In quadrupeds, particularly horses and ungulates the term is commonly used to refer to the carpus, probably because of its similar hinge or ginglymus action. The joints between the femur, tibia and patella are known as the stifle in quadrupeds. In insects and other animals the term knee is used widely to refer to any ginglymus joint. See also * Cartilage * Knee Cartilage Replacement Therapy * Knee examination * Kneebar * Reflex hammer * Partial knee replacement Additional images Image:Knie ct.gif|Knee MR Image:Knie mr.jpg|Knee MR Image:Knie-roentgen-r-seite.jpg|Knee X-ray Image:Legamenti crociati.jpg|Cruciate ligaments Image:Gray348.png|Left knee-joint from behind, showing interior ligaments. Image:Gray351.png|Capsule of right knee-joint (distended). Lateral aspect. Image:Knee skeleton lateral anterior views.svg|Anterior and lateral view of knee. Notes References * * * * * * * *Knee